The present invention relates to a semiconductor memory device for storing electrical charges as information in a capacitive insulating film made of a material with a high relative dielectric constant, and also relates to a method for fabricating the same.
As the multimedia applications have been broadening in recent years, it has become more and more necessary to store and process an increasing quantity of digital information using various types of information processors. Under the circumstances such as these, the quantity of information to be processed per unit time, i.e., the capacity of each single semiconductor memory device used for storing that information thereon, would continue to rise by leaps and bounds. However, in a dynamic random access memory (DRAM), for example, as the required capacity increases, the charge storage capacity of each cell should be maintained at an approximately conventional level (i.e., about 30 fF), even though the size of each cell should be reduced to satisfy a downsizing requirement. Accordingly, to ensure a sufficient capacity and reduce the size of a cell at the same time, various materials with a high relative dielectric constant, such as barium strontium titanate (BST), have lately been applied to the capacitive insulating film of a memory cell more and more often. In this specification, a capacitive insulating film of this type, which is made of a material with a high dielectric constant and in which electrical charges can be stored as information, will be called a "charge-storable dielectric film".
Nevertheless, since a BST film is usually in the form of a polycrystalline thin film, it is important to suppress the leakage current flowing therethrough.
To solve such a problem, a "multi-layered dielectric film" is used as disclosed in Japanese Laid-Open Publication No. 7-161833, for example.
FIG. 8 illustrates a cross-sectional structure of a charge-storable dielectric film disclosed in the publication identified above. As shown in FIG. 8, this structure includes: a titanium (Ti) film 101; a titanium dioxide (TiO.sub.2) film 102; a high-dielectric-constant film 103 made of (Ba, Pb)(Zr, Ti)O.sub.3 ; a TiO.sub.2 film 104; and a Ti film 105, which are stacked in this order on a substrate 100.
In this conventional DRAM structure, a charge-storable dielectric film is formed by interposing a pair of TiO.sub.2 films 102 and 104 between the high-dielectric-constant film 103 and the Ti films 101 and 105 functioning as respective electrodes, thereby suppressing the leakage current flowing through the high-dielectric-constant film 103. A smaller amount of current leaks through the TiO.sub.2 films 102 and 104 as compared with the (Ba, Pb)(Zr, Ti)O.sub.3 film 103, and the TiO.sub.2 films 102 and 104 have a higher dielectric breakdown voltage than the (Ba, Pb)(Zr, Ti)O.sub.3 film 103. Accordingly, a charge-storable dielectric film can be formed with a leakage current reduced in total.
However, in this conventional DRAM structure, the high-dielectric-constant film 103, which is originally intended to be a capacitive insulating film, is sandwiched between the TiO.sub.2 films 102 and 104 to reduce the leakage current. As a result, the relative dielectric constant of these three insulating films decreases in total. This is because the relative dielectric constant of the TiO.sub.2 film is as low as about 25. Also, the total relative dielectric constant of these three insulating films interposed between the upper and lower Ti films 101 and 105 is calculated the same way as the capacitance of a circuit in which three capacitors, formed by these insulating films, are connected in series to each other. Thus, the DRAM structure shown in FIG. 8 and disclosed in the above-identified document has decreased charge storability per unit area. This is one of the factors interfering with the downsizing of an overall memory structure.